Composition and process for preventing electrostatic discharge

ABSTRACT

The invention provides for a process of using a static dissipative, polymeric composition to prevent electrostatic discharge. The static dissipative, polymeric composition is both humidity independent and water insoluble. The present invention also provides an article comprising a substrate to be protected from electrostatic discharge.

The present application is a divisional of U.S. Ser. No. 08/002,709filed on Jan. 11, 1993 which matured into U.S. Pat. No. 5,320,780 onJun. 14, 1994.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a composition and a process for the preventionof electrostatic discharge (ESD) by-providing a static dissipativepolymeric composition and contacting it with a substrate.

BACKGROUND OF THE INVENTION

Electrostatic discharge (ESD) is one of the most destructive phenomenain modern industry, especially the electronics industry. Electrostaticdischarge is defined in the U.S. Military Handbook DOD-HKPK-263 as atransfer of electrostatic charges between bodies at different potentialsby direct contact or induced by an electrostatic field. Electrostaticcharge can build up on non-conductive materials as the result of thecapture or release of electrons. A non-conductive material can captureor release an electron by rubbing or heating it, or it can becomecharged through contact with another previously charged object. Forexample, if a non-conductive material captures an electron from one ofthe sources listed above, it will become negatively charged and willhold that charge. If a positively charged body comes into contact withthe negatively charged non-conductive material or comes within thestatic field, an arc may be discharged from the non-conductive materialto the positively charged material in order to dissipate the buildup ofthe electrical charge. This transfer of electrostatic charge causesdestruction and damage to electronic components estimated at millions ofdollars a year.

One of the primary generators of ESD is the movement of personnel orequipment across the floor in a manufacturing area, assembly area orshipping area. Static damage to integrated circuit components byoperating personnel is becoming one of the most significant problemsplaguing the electronics industry. Table 1 below represents typicalelectrostatic voltages generated for various types of movement:

                  TABLE 1                                                         ______________________________________                                        MEANS OF       ELECTROSTATIC VOLTAGES                                         STATIC         10-20% Relative                                                                            65-90% Relative                                   GENERATION     Humidity     Humidity                                          ______________________________________                                        Walking across carpet                                                                        35,000       1,500                                             Walking over vinyl floor                                                                     12,000       250                                               Worker at bench                                                                               6,000       100                                               Mobile storage carts                                                          On vinyl floors                                                                               5,000                                                         ______________________________________                                    

Frequently, little attention is given to the ESD control of a floor orworkbench that is near or adjacent to an ESD controlled area. It is notuncommon to assume that operating personnel will be sufficientlygrounded with a wrist strap and therefore, the floor or workbench neednot be grounded or protected from ESD. However, various non-operatingpersonnel and equipment that may not be sufficiently grounded move inand out of work areas and interact with operating personnel, thuscreating ESD and the destruction of sensitive electronic components.

As device technology advances to achieve higher speeds and greaterfunctional density, device geometries are decreasing, line widths andspaces are getting smaller and oxide layers are getting thinner. As aresult, lower voltage and current will damage sensitive electroniccomponents. Although electronic components are most vulnerable at thechip or integrated circuit stage, when numerous integrated circuits areassembled on a board or even a final product, damage can occur. Table 2represents the susceptibility ranges of various electronic devices:

                  TABLE 2                                                         ______________________________________                                                     RANGE OF ESD SUSCEPTIBILITY                                      DEVICE TYPE  (VOLTS)                                                          ______________________________________                                        Bipolar transistors                                                                        380-7,000                                                        CMOS         250-3,000                                                        EPROM        100                                                              Film resistors                                                                             300-3,000                                                        (thick, thin)                                                                 GaAsFet      100-300                                                          MOSFET       100-300                                                          OP-AMP       190-2,500                                                        SAW          150-500                                                          Schottky diodes                                                                            300-2,500                                                        Schottky TTL 1,000-2,500                                                      SCR          680-1,000                                                        VMOS          30-1,800                                                        ______________________________________                                    

In light of the potential damage that ESD can cause to electroniccomponents, various ESD protective coatings have been developed to beapplied to non-conducting substrates to make them conductive or staticdissipative. A substrate that is conductive has an electricalresistivity in the range of about 10⁰ to about 10⁵ ohms/square, whereasa substrate that is static dissipative has an electrical resistivity inthe range of about 10⁵ to about 10¹² ohms/square. Within the conductiveor static dissipative range, static potentials may be dissipated withoutharming the electronic component. However, a substrate with anelectrical resistivity in the conductive range may pose harm tooperating personnel. Further, a substrate with an electrical resistivityof greater than 10¹² ohms/square is considered to be insulative andhighly destructive with respect to electronic components. Therefore, itis desirable to have an ESD protective coating that is within the staticdissipative range.

The most common type of ESD protective coating in the electronicsindustry is based on ionic conduction, like that disclosed in GreatBritain Patent No. 2,148,915A to Berbeco. Ionic conduction involves ahumidity dependent coating in which small amounts of moisture are neededto allow for the migration of ions and hence the overall flow ofelectrons. Ionic conduction based coatings are conductive (10⁰ to 10⁵ohms/square) or static dissipative (10⁵ to 10¹²).

However, there are shortcomings to ionic conduction based ESD protectivecoatings. For example, these coatings are humidity dependent. Ionicprotective coatings are made up of molecules that have an atom with apositive or negative charge on one end of the molecule, such as a sodiumor chlorine atom, which attracts water molecules. When the humidity ishigh, the abundance of water molecules will be attracted to and willbuild up on the protective coating. When water molecules are allowed tobuild up over the surface of the ionic coating, the innate conductivityof the water film dissipates the buildup of any accumulated electricalcharge, thereby preventing an unwanted electrical static discharge.However, if the humidity drops below 20%, ionic conduction isdrastically minimized such that operating technicians, just walkingacross the floor to the workbench, will build up a static electriccharge sufficient to damage an electronic component (See Tables 1 and2). Another disadvantage to humidity dependent ionic conduction is thatwhen the humidity decreases to about 20%, the air itself, being dry,becomes part of the electrostatic buildup mechanism every time there isan air flow (e.g., wind, air conditioning, blower) passing over aninsulated substrate.

Yet another disadvantage of ionic conduction protective coatings is thatthey are easily degraded by contact with water or detergent solutions.The protection provided by an ionic conduction based protective coatingis greatly reduced if the coated substrate is washed with water or adetergent solution. Therefore, the substrate in need of protection mustbe constantly recoated.

As will be discussed below, the present invention has the technicaladvantage of providing ESD protection without concern for the level ofhumidity. Because the present invention is water insoluble, it has thetechnical advantage of allowing coated substrates (e.g., floors,workbenches, etc.) to be washed or cleaned without removing the ESDprotective coating. Moreover, the composition can be applied to objectswith identification numbers, etc. because it forms a clear film whendried.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a staticdissipative self-protonating polymeric composition that is both humidityindependent and water soluble. The polymeric composition comprises aprotonic acid self-protonating conducting polymer, a non-ionicsuffactant, an acrylic emulsion, a soft polyethylene emulsion, a hardpolyethylene emulsion, a glycol ether, and at least one plasticizer inan aqueous solution. The protonic acid self-protonating conductingpolymer component is an aromatic unsaturated cyclic hydrocarboncontaining repeating units of one or more cyclic rings bridged with anitrogen amine and repeating units of the same ring systems bridged withnitrogen imines. The aromatic unsaturated ring systems which make up thebackbone structure of the polymer can be unsaturated rings systems, suchas benzenes, cyclopentadienyls, naphthalenes or indenes. Preferably,however, the protonic acid self-protonating conducting polymer is eithera phosphated or sulfonated polyaniline, and more preferably, it is asulfonated polyaniline.

Once the composition is applied to a substrate and allowed to dry, itprovides ESD protection by providing a static dissipative polymericcomposition that is humidity independent and water insoluble thatprovides a static dissipative level of ESD protection in the range offrom about 10⁵ to about 10¹² ohms/square over the surface of thesubstrate.

In one aspect of the present invention, a composition for preventingelectrostatic discharge (ESD) is offered which provides a staticdissipative polymeric composition that is humidity independent, waterinsoluble and clear. In this preferred embodiment, the staticdissipative polymeric composition comprises a non-ionic surfactant, asulfonated polyaniline, an acrylic emulsion, soft polyethylene emulsion,hard polyethylene emulsion, glycol ether, and at least one plasticizerin an aqueous solution. When contacted with the substrate and allowed todry, it provides the surface of the substrate with an electricalresistivity in the range of about 10⁵ to about 10¹² ohms/square.

In another aspect, the present invention provides for an articlecomprising a substrate that has been contacted with the staticdissipative polymeric composition. The substrate, after being contactedwith the static dissipative polymeric composition, has an electricalresistivity of 10⁵ to 10¹², thus allowing for the protection ofelectronic components from ESD without detrimentally exposing operatingpersonnel to health risks.

A long-felt need exists for a static dissipative ESD protective coatingthat is humidity independent, water insoluble and preferably clear.Accordingly, one aspect of the present invention discloses a compositionand a process for preventing ESD by providing a static dissipative,non-ionic polymeric composition that is humidity independent, waterinsoluble and clear. In accordance with the present invention, ESD isprevented by contacting a substrate to be protected, such as concrete,vinyl or wooden floors, metal and wooden workbenches, or any insulativesubstrate that has a potential to cause damage to electronic components,with a static dissipative, non-ionic polymeric composition.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a static dissipative polymericcomposition for preventing Electrostatic Discharge (ESD) and a processfor contacting the composition to a substrate to be protected from ESDwith the polymeric composition. The static dissipative polymericcomposition of the present invention comprises a protonic acidself-protonating conducting polymer, a non-ionic surfactant, an acrylicemulsion, a soft polyethylene emulsion, a hard polyethylene emulsion,glycol ether and at least one plasticizer in an aqueous solution. Onceapplied to the substrate and allowed to dry, the composition provides acoating that is humidity independent and water insoluble. It is theself-protonating polymer which gives the composition the humidityindependence which sets it apart from static dissipative compositionsfound in the prior art. As previously discussed, the static dissipativecompositions of the prior art are usually in a cationic or anionic formhaving a hydrophilic end that attracts airborne water molecules. As aresult, a conducting film of water forms over the surface of thesubstrate. Thus, the effectiveness of these static dissipativecompositions varies with the level of humidity that is present in theair. In contrast, the static dissipative composition of the presentinvention, for reasons stated above, does not require water molecules tobe conductive and is thus humidity independent. After the composition isallowed to dry, the substrate has an electrical resistivity in the rangeof about 10⁵ to about 10¹² ohms/square.

The non-ionic surfactant used in the polymeric composition may be alinear ethoxylated alcohol, a propoxylated alcohol, nonylphenolethoxylate, octyl phenol ethoxylate, alkyl polyoxyalkylene ether, abranched polyoxyalkylene fatty alcohol, a branched ethoxylated alcoholor a polyoxyalkylene alcohol. The non-ionic surfactant may be an octylphenol ethoxylate or a nonylphenol ethoxylate with the latter being thepreferred non-ionic surfactant. The percentage by weight of thenon-ionic surfactant to the total weight of the composition ispreferably in the range from about 0.1% to about 1.0%, and morepreferably is 0.16%.

While the protonic acid self-protonating conducting polymer in thepreferred embodiment of the present invention is a sulfonatedpolyaniline, it should be understood that other aromatic conductingpolymer systems may be used in place of polyaniline as the staticdissipative conducting component. For example, the protonic acidself-protonating conducting polymer component may be any aromaticunsaturated cyclic hydrocarbon with repeating units of one or more ringsbridged with a nitrogen amine and repeating units of the same ringsystems bridged with a nitrogen imine, as illustrated in FIG. 1.##STR1##

In addition, the ring systems may have an anionic group such as asulfonate group or a phosphate group bonded to it. In fact, it is morecommon for every other ring system to have an anionic group bonded toit, as illustrated in FIG. 2. ##STR2##

The aromatic unsaturated ring systems which make up the backbonestructure of the polymer can be unsaturated rings systems, such asbenzene, cyclopentadienyls, naphthalenes or combinations of those ringsystems. Preferably, the anionic substituents attached to those ringsystems is either a phosphate group or a sulfonate group, the protonicacid self-protonating conducting polymer is either a phosphated orsulfonated polyaniline with a percentage weight to the total weight ofthe composition in the range from about 1% to about 5%, and preferablyis about 2%. More preferably, however, the self-protonating conductingpolymer is a sulfonated polyaniline like that disclosed in "Synthesis ofSelf-Doped Conducting Polyaniline," J. Am. Chem. Soc. 1990 112,2800-2801, which is incorporated herein by reference.

When in the solution phase, the self-protonating polymer has differentchemical properties than it does when the aqueous solution dries. In thepreferred embodiment, the sulfonated polyaniline, and resonant formsthereof, is insoluble in water (See FIG. 2). Thus, to make thepolyaniline soluble with the other components in the aqueous solution,the polyaniline is reacted with ammonia hydroxide to form thecorresponding ammonium salt that is associated with the sulfonate groupon the aromatic ring, as illustrated in FIG. 3. ##STR3##

The presence of the ammonium salt makes the polyaniline soluble in waterwhich allows it to be readily mixed with the other compositioncomponents. After the composition has been applied to the substrate, itbegins to dry. As it drys, the ammonium is driven off and thepolyaniline immediately protonates the imine nitrogen atoms to give thepolymer its conductive characteristics that are independent on theamount of humidity present in the air.

The acrylic emulsion of the above polymeric composition consists of anacrylic polymer with metal cross-linking in a water and glycol ethersolution. The acrylic polymer may be a 40% copolymer consisting of ethylmethacrylate, methyl methacrylate, styrene and isobutyl methacrylate.Preferably, the percentage weight of the acrylic emulsion to the totalweight of the composition is in the range from about 30% to about 40%,and more preferably is about 35.91%.

The soft polyethylene emulsion of the above polymeric composition may bea 30-40% low molecular weight solid polyethylene emulsion thatconstitutes from about 1.0% to about 10.0% by weight of the totalcomposition, and more preferably is about 5.06% by weight of the totalcomposition.

The hard polyethylene emulsion of the above polymeric composition may bea 30-40% high molecular weight solid polyethylene emulsion thatconstitutes from about 1.0% to about 10.0% by weight of the totalcomposition, and more preferably is about 1.69% by weight of the totalcomposition.

Another ingredient used in the present composition is a glycol ether.The glycol may be selected from the group consisting of any dihydric orpolyhydric alcohol. Preferably, the alcohol is an aliphatic dihydricalcohol which is selected from the group consisting of ethanediol,propanediol, butanediol, or pentanediol. More preferably, the glycolether is a diethylene glycol monomethyl ether. The percentage by totalweight of the composition of the glycol ether ranges from about 1.0% toabout 7.5 and more preferably, is about 4.32%.

Yet another ingredient used in the present composition is at least oneplasticizer. Preferably, the first plasticizer, plasticizer A, of theabove polymeric composition is a phthalate ester, including benzylbutylphthalate, dibutyl phthalate, dioctyl phthalate, or benzyloctyl orcompositions similar thereto. More preferably, however, plasticizer A isa benzylbutyl phthalate. A second plasticizer, plasticizer B, may alsobe used and it is preferred that a second plasticizer be used. When asecond plasticizer is used, the preferred compound is a tributoxyethylphosphate. The preferable concentration of the first plasticizer, A, isin the range of from about 0.9% to about 2% by total weight of thecomposition and more preferably is 0.97% by weight. Likewise, thepreferred concentration of the second plasticizer, B, is in the range offrom about 0.9% to about 2% by total weight of the composition with themore preferred concentration being 0.97%.

The aqueous portion of the composition is water and preferably,deionized water, which comprises from about 40% to about 60% by totalweight of the composition, and more preferably, comprises about 49.12%.

The static dissipative polymeric composition of the present invention isnon-ionic, and therefore, humidity independent. Furthermore, the staticdissipative polymeric composition of the present invention is waterinsoluble with a pH of about 8.80 to about 9.00, thus allowing for thecleaning or washing of a substrate to be protected without removing thestatic dissipative polymeric composition from the substrate. The staticdissipative polymeric composition of the present invention can bemanually or robotically applied to a substrate to be protected, such asa floor, workbench, integrated circuit (IC) rails, or tote boxes.

The present invention also provides for an article comprising asubstrate to be protected from ESD. The substrate, after being contactedwith the static dissipative polymer composition of the presentinvention, has an electrical resistivity in the range of about 10⁵ toabout 10¹² ohms/square. The above substrate can be a floor surface suchas a wooden floor, a concrete floor, a vinyl floor, a wooden, plastic ormetal workbench, IC rails, metal tote boxes or similar substrates thatneed to be protected from ESD.

EXAMPLE

Ammonium peroxydisulfate, (NH₄)₂ S₂ O₈ (11.5 g, 0.0504 mole) wasdissolved in 200 ml of 1M HCL which had been precooled to 1° C. Aniline(20.0 ml, 0.219 mole) was dissolved in 300 ml of 1M HCL which had beenprecooled to 1° C. The aniline solution was placed in a 750 mlErlenmeyer flask with a magnetic stirring bar and the container wasplaced in an ice bath on a magnetic stirring plate.

The (NH₄)₂ S₂ O₈ solution was added to the aniline solution, withconstant stirring, over a period of 1 minute. The solution was thenstirred in an ice bath for ˜1.5 hours during which time the temperatureremained below 5° C. Three to 5 minutes after the reactants were mixed,the solution started to take on a blue-green tint and then becameintense blue-green with a coppery glint as a precipitate formed. Thecoppery glint was less pronounced after ˜1 hour.

After ˜1.5 hours, the precipitate was collected on a Buchner funnel(diameter 7.5 cm) using a water aspirator. The precipitate cake waswashed portion-wise (60 ml/portions) with 1M HCL until the initiallypale violet filtrate became colorless. The liquid level was constantlyadjusted so that it remained above the top of the precipitate. Thisprevented cracking of the precipitate cake, which would result ininefficient washing of the precipitate. A minimum of 500 ml of 1M HCLwas used, this "as-made" precipitate is polyaniline in the incompletelyprotonated emeraldine hydrochloride form.

After the above washing, the precipitate remained under suction for ˜10minutes until significant cracking of the moist filter cake occurred. Itwas then transferred on the filter paper to a vacuum desiccator anddried under dynamic vacuum for ˜2 hours.

The moist emeraldine hydrochloride precipitate cake obtained after the10-minute suction treatment in the Buchner funnel described above wassuspended with constant stirring in 500 ml 0.1M NH₄ OH solution. If,after 10 minutes, the pH of the suspended liquid was <8, 1.0M NH₄ OH wasadded drop-wise to bring the pH up to ˜8. The suspension was stirred for˜15 hours. The powder was collected on a Buchner funnel (diameter 7.5cm) and was washed with 500 ml of 0.1M NH₄ OH in 60 ml portions,precautions to avoid cracking of the filter cake being taken asdescribed above. The powder was resuspended in an additional 500 ml of0.1M NH₄ OH and was stirred for 1 hour, collected on a Buchner funneland washed with 500 ml of 0.1M NH₄ OH in 60 ml portions. The powder waspartially dried under suction on a Buchner funnel for ˜10 minutes. Themoist emeraldine base powder was then transferred on the filter paper toa desiccator and was dried under dynamic vacuum for ˜4 hours.

Emeraldine hydrochloride powder (I) was synthesized from an aqueoussolution of aniline, (NH₄) S₂ O₈, and HCL. It was then convened toanalytically pure emeraldine base (II) via the previously describedmethod. Emeraldine base (0.5 g) was sulfonated by being dissolved in 40mL of fuming sulfuric acid with constant stirring. The color of thesolution changed from dark purple to dark blue during ˜2 hours at roomtemperature. The solution was then slowly added during ˜20 minutes to200 mL of methanol to precipitate most of the product, the temperaturebeing held between 10° and 20° C. by an ice bath. Precipitation wascompleted by the addition of 100 mL of acetone. The green powder wasthen collected on a Buchner funnel, and the precipitate cake was washedat least 10 times with ˜50-mL portions of methanol until the filtratehad a pH of 7 when tested by wet pH paper. The liquid level in theBuchner funnel was constantly adjusted so that it remained above the topof the precipitate in order to prevent cracking of the precipitate cake,which would result in inefficient washing. It was then permitted toremain under suction for approximately 10 minutes; the resultingprecipitate cake was slightly soluble in water, giving a green solution.The filter cake then was transferred on the filter to a vacuumdesiccator and dried under dynamic vacuum for 24 hours.

Deionized water, non-ionic surfactant and the sulfonated polyanilinewere all charged into a mixing vessel and stirred easily at roomtemperature. The pH was raised to 8.80 with 28% ammonium hydroxide andallowed to stir for at least 20 minutes. Acrylic emulsion, softpolyethylene emulsion, hard polyethylene emulsion, glycol ether,tributoxyethyl phosphate and phthalate ester were added one by one inthe order listed allowing a 10-minute mixing time between eachingredient. The pH was checked again to make certain that it was between8.80-9.00. The composition was then applied to a floor with a vinyl tilesurface with a cotton string mop and was also applied to IC rails, toteboxes and other surfaces and was allowed to dry for 1.0 to 1.5 hourswhich gave the floor a resistivity of 5×10⁸ ohms/square.

Various modifications are contemplated which will be apparent to thoseskilled in the art and can be applied to the preferred embodimentdescribed above without departing from the spirit and scope of theinvention as defined by the following claims.

I claim:
 1. A process for preventing electrostatic discharge, comprising the steps of:(a) providing a static dissipative polymeric composition with a conductivity, said static dissipative polymeric composition comprising: a non-ionic surfactant; a sulfonated polyaniline; an acrylic emulsion; a soft polyethylene emulsion; a hard polyethylene emulsion; a glycol ether, and at least one plasticizer, wherein the polymeric composition is in an aqueous solution; (b) contacting a substrate with said static dissipative polymeric composition, whereby said substrate, after being contacted with said static dissipative polymeric composition, has an electrical resistivity in the range of about 10⁵ to about 10¹² ohms/square.
 2. A method as in claim 1, wherein the conductivity of said static dissipative polymeric composition is humidity independent.
 3. A method as in claim 1, wherein the pH of said static dissipative polymeric composition is in the range of about 8.80 to about 9.00.
 4. An article comprising a substrate, said substrate having been contacted with the static dissipative polymeric composition of claim
 1. 5. An article as in claim 4 wherein said substrate has an electrical resistivity in the range of about 10⁵ to about 10¹² ohms/square.
 6. An article as in claim 4, wherein said substrate is a workbench.
 7. An article as in claim 4, wherein said substrate is a floor.
 8. An article as in claim 4, wherein said substrate is an integrated circuit rail.
 9. An article as in claim 4, wherein said substrate is a tote box.
 10. A process for preventing electrostatic discharge, comprising the steps of:(a) providing a static dissipative polymeric composition, said static dissipative polymeric composition comprising: a sulfonated polyaniline which comprises about 2% by weight of the composition; a nonylphenol ethoxylate which comprises about 0.16% by weight of the composition; an acrylic emulsion consisting of ethyl methacrylate, methyl methacrylate, styrene and isobutyl methacrylate which comprises about 35.91% by weight of the composition; a low molecular weight soft polyethylene emulsion comprising about 5.06% by weight of the composition; a high molecular weight hard polyethylene emulsion comprising about 1.69% by weight of the composition; a diethylene glycol monomethyl ether comprising about 4.32% by weight of the composition; a first plasticizer benzylbutyl phthalate comprising about 0.97% by weight of the composition; a second plasticizer tributoxyethyl phosphate comprising about 0.97% by weight of the composition, in an aqueous solution which comprises about 49.12% by weight of the composition; (b) contacting a substrate with said static dissipative polymeric composition, whereby said substrate, after being contacted with said static dissipative polymeric composition, has an electrical resistivity in the range of about 10⁵ to about 10¹² ohms/square. 